Germination Temperatures for Sowing Seeds
Hardy vegetables, such as lettuces, need a minimum temperature of 5-7°C (41-45°F) to germinate. Tomatoes and marrows require 10-13°C (50-55°F), cucumbers 21°C (70°F) and aubergines 18°C (64°F).
These higher temperatures are attainable in a propagator, and can be achieved in the greenhouse with a little ingenuity. If the heater is placed under the staging, and a polythene tent erected over the seed pans, an adequate and fairly steady temperature can be maintained.
Even without a heater you can still raise many plants in a greenhouse. It simply means that you will have to sow later, thus losing some of the advantage of protected sowing.
For instance, tomatoes can be sown in February — if you can provide moderate heat for germination. Without heat, sowing must be delayed until late March or April.
Sowing in a garden frame can begin at the same time as in a cold greenhouse. Conserve warmth as much as possible, ventilating at mid-day on warm days and covering with sacks or other material on cold nights. A soil-heating cable may be installed in a frame, in which case sowing times are as for a heated greenhouse.
Sowing in pans and trays
Fill the container to the rim with seed compost and press it down with the bottom of a similar container until it is 1/2 in. (12 mm) below the rim.
Sprinkle the seeds very thinly and evenly, spacing larger seeds, such as tomatoes, and pelleted seeds at least in. apart.
Cover the seeds with 8 in. (3 mm) of compost and firm the surface evenly. Water lightly, preferably using a fine rose that projects the spray upwards.
Use a crayon or felt-tipped pen to write the name of the variety and the date of sowing on the side of the tray or on a label.
If you are germinating the seeds in a greenhouse, but without a propagator, cover the container with a sheet of glass to conserve moisture and place a piece of brown paper or card over the top.
If growing on a window-sill, place each tray or pot in a polythene bag, tying it tightly at the top to conserve moisture but enclosing as much air as possible.
Remove the covering as soon as the first shoots emerge. If left in the dark they can become drawn and worthless in 24 hours.
In a greenhouse propagator, remove the tray as soon as the seedlings break the surface. Place the seedlings where they will get both warmth and light.
Sowing in pots
Seeds of marrows, cucumbers, melons and sweet corn are best sown in potting compost in 3in (75mm) plastic, clay or peat pots and grown on without disturbance until their final planting.
Sow two seeds 1in. (12mm) apart and in. deep in the centre of the pot, inserting the seeds of marrows, melons and cucumbers on edge. Keep covered with glass and brown paper until they germinate.
In most cases two seedlings appear. Remove the weaker one.
French and runner beans can be given a good start by sowing them in moist peat in plastic cream cartons, in which drainage holes have been punched. Put five or six seeds 3/4in. (20mm) deep in each carton. Seal them in a plastic bag and put the containers in an airing cupboard for a few days until the seeds germinate.
Just as seedlings outdoors need thinning, so must those in trays be given room to develop. Do this by spacing them out in other trays or planting them singly in pots. Make this move, known as ‘pricking out’, as soon as the seedlings are large enough to handle and before they become tall and intertwined.
Fill the trays or pots with John Innes No. 1 potting compost or a soil-less compost to within 1/2in. (12mm) of the rim when made firm and level.
Lift each seedling with the point of a plastic plant label, holding it by a leaf and not by the stem. Use a pencil or stick to make a hole in the compost and plant the seedling with its stem covered to just below the seed leaves — the leaves that appear first as the seedling breaks the surface.
Space lettuces and other small plants 11in (40 mm) apart. Allow 21in (65 mm) between tomatoes if pricked out into trays, though it is better to prick them out individually into 3in (75 mm) pots.
Water the seedlings and keep them out of direct sunlight for two days, then place them close to the glass to encourage sturdy, short-jointed growth. Water regularly and shade the glass during sunny weather.
Plants raised in a greenhouse or indoors must be acclimatised before being planted outdoors.
Ideally, place them in a garden frame or under cloches a week or so before planting out. At first, leave frame lights or cloches open only during the day. A few days later, give some ventilation at night and, finally, leave the protection off altogether.
If a frame or a cloche is not available, stand the plants in a porch or verandah by day and take them in for the first few nights.
Remember that plants in small containers exposed to sun and wind need more frequent watering than when under cover.
Sowing seeds in a propagator
Propagators promote the ideal conditions for seeds to germinate: constant warmth and humidity. They help in producing crops which can be harvested days, or even weeks, earlier, while similar produce in the shops is still expensive.
The simplest consists of a seed tray with a snugly-fitting, plastic dome. This type is placed either above a greenhouse heater or in a warm spot indoors.
Most propagators, however, are electrically heated, some by low voltage cables set in sand, others by a warming panel on which the domed seed tray is placed.
After the initial outlay, an electric propagator costs only a few pence a week to run. A small version suitable for a window-sill consumes merely 16 watts and a large greenhouse type, measuring 36 x 20 in. (1m x 510 mm), only 40 watts.